Control device of automatic transmission

ABSTRACT

In a process wherein a before-shifting gear ratio Gx changes to a gear ratio of the target gear position, an actual shift time is measured by regarding as a shift starting point an earlier one between a time point when the present gear ratio G has repeated decreasing by a value equal to or larger than a predetermined unit gear ratio ΔG a predetermined number of times N and a time point when the present gear ratio reaches a first gear ratio G1 that is set to a value smaller than the gear ratio Gx by a predetermined value. If the gear ratio changes rapidly, the start of shifting can be detected at a time ts1 when the present gear ratio G reaches the gear ratio G1, and if the gear ratio changes slowly, the start of shifting can be detected at a time ts4 when the gear ratio has repeated decreasing by a value equal to or larger than a predetermined unit gear ratio a predetermined number of times N. In either case, the start of shifting can be detected early at a point in proximity to a shift starting point ts. Further, a time point when the present gear ratio G reaches a second gear ratio, which is set to a value larger than the gear ratio Gx by a predetermined value, is also regarded as the shift starting point, so that the start of shifting can be detected early even in the case where an engine races. This enables accurate calculation of a shift time in shift time learning control for controlling working fluid pressure in an automatic transmission.

TECHNICAL FIELD

[0001] This invention relates generally to a control device of anautomatic transmission for vehicles, and more particularly to a controldevice of an automatic transmission that provides learning control toenable completion of shifting in a predetermined period of time.

BACKGROUND ART

[0002] An automatic transmission changes gear positions by selectivelyactuating friction engagement elements such as clutches and brakes of atransmission gear mechanism by oil pressure. In this case, after theoutput of a shifting command, it takes much time to supply oil pressureto the selected friction engagement elements and the like, and thus, thestart of the actual shifting in which the friction engagement elementsstart operating is delayed from a time indicated by the shiftingcommand.

[0003] To change gear positions smoothly and quickly when controlling anautomatic transmission, Japanese Laid-Open Patent Publication No.1-193445, for example, proposes correcting an oil pressure in the nextshifting based on the present shift time by learning control so that theactual shift time can be equal to a predetermined time.

[0004] Conventionally, to measure the shift time, a shift startdetermination gear ratio GI is set in advance between a before-shiftinggear ratio (the gear ratio of a before-shifting gear position) Gx and anafter-shifting gear ratio (the gear ratio of a after-shifting gearposition) Gy. A time point when the sequentially detected gear ratiocrosses the shift start determination gear ratio G1 is regarded as atime point when the shifting is started, and a period of time until thegear ratio reaches a shift end determination gear ratio G3 set in thesame manner as the gear ratio G1 is regarded as the shift time.

[0005] Incidentally, since the actual shifting starts when the gearratio starts changing even at a low rate, the above-mentioned shiftstart determination gear ratio G1 is preferably set to a value as muchclose to the before-shifting gear ratio Gx as possible. Actually,however, the gear ratio G1 is set to a value smaller than thebefore-shifting gear ratio by a predetermined value in terms ofdispersion in detection.

[0006] However, if the shift start determination gear ratio G1 is set toa value that is smaller than the before-shifting gear ratio Gx to adegree as mentioned above, the measured time is affected by a period oftime until the gear ratio G1 is reached.

[0007] If, for example, an oil pressure in shifting is low and a changein gear ratio is extremely small at the start of shifting as indicatedby “a” in FIG. 6, there is a great difference between the actual shifttime and the measured shift time. Even if the measured time is learned,it is impossible to properly correct an oil pressure in the nextshifting.

[0008] More specifically, if it is determined that the measured shifttime is shorter than a predetermined of time even though the actualshift time is longer than the predetermined period of time, the oilpressure is corrected to decrease in the next shifting. This may cause ashift shock.

[0009] On the other hand, if an engine races during shifting due to theunmatched engagement and disengagement of the friction engagementelements as shown by “b” in FIG. 6, the same phenomenon occurs since ittakes much time for the gear ratio to cross the shift startdetermination gear ratio.

[0010] As stated above, in the conventional way of measuring the shifttime, it is impossible to measure the actual shift time, which isimportant in learning control, with desirable accuracy.

[0011] It is therefore an object of the present invention to provide acontrol device of an automatic transmission, which is capable of findinga shift time with high accuracy to provide proper learning control evenin the case where a gear ratio changes at an extremely low rate at thestart of shifting or in the case where an engine races.

DISCLOSURE OF INVENTION

[0012] To attain the above-mentioned object, according to the firstaspect of the present invention, there is provided a control device ofan automatic transmission, which selectively supplies hydraulic oilpressure to a plurality of friction engagement elements to change aplurality of gear positions by engaging and disengaging the frictionengagement elements in combinations, the control device comprising: anoil pressure regulating actuator for regulating the hydraulic oilpressure; shift time learning control means for performing shift timelearning control by comparing a target shift time and an actual shifttime to thereby control the oil pressure regulating actuator; andwherein the shift time learning control means comprises gear ratiocalculating means for finding a present gear ratio, and shift timemeasuring means for measuring the actual shift time by determining thatshifting is started if the present gear ratio has repeated decreasing bya value equal to or larger than a predetermined unit gear ratio apredetermined number of times.

[0013] In the shift time learning control for controlling hydraulic oilpressure in the automatic transmission, since the actual shift time ismeasured by regarding as a shift starting point a time point when thepresent gear ratio has repeated decreasing by a value equal to or largerthan a predetermined unit gear ratio a predetermined number of times, itis possible to surely detect the start of shifting without fail evenwhen the shifting proceeds slowly due to a low hydraulic oil pressure.In particular, a smaller unit gear ratio enables the start of shiftingto be detected earlier at a time point closer to the actual shiftstarting point. This improves the shift time measuring accuracy, andthus provides proper shift time learning control without causing a shiftshock.

[0014] According to the second aspect of the present invention, theactual shift time is measured by regarding as a shift starting point anearlier time point between a time point when the present gear ratio hasrepeated decreasing by a value equal to or larger than a predeterminedunit gear ratio a predetermined number of times and a time point whenthe present gear ratio reaches a first gear ratio that is set to a valuesmaller than a before-shifting gear ratio by a predetermined value.

[0015] By determining not only whether or not the present gear ratio hasrepeated decreasing by a value equal to or larger than a predeterminedunit gear ratio a predetermined number of times but also whether or notthe present gear ratio has reached the first gear ratio, it is possibleto detect the start of shifting before the present gear ratio reachesthe first gear ratio as a result of sequential decreases within therange of the unit gear ratio or more in the case where the shiftingproceeds slowly, and it is possible to early detect the start ofshifting since the present gear ratio quickly reaches the first gearratio in the case where the shifting proceeds fast.

[0016] According to the third aspect of the present invention, theactual shift time is measured by regarding as a shift starting point atime point when the present gear ratio reaches a second gear ratio thatis set to a value larger than a before-shifting gear ratio by apredetermined value.

[0017] In the case of shifting in which an engine races after the startof the actual shifting, it is possible to detect the racing of theengine if the present gear ratio has reached the second gear ratio andto detect the start of shifting at a time point in proximity to theactual shift starting point.

[0018] According to the fourth aspect of the present invention, theactual shift time is measured by regarding as a shift starting point anearlier time point between a time point when the present gear ratioreaches a first gear ratio that is set to a value smaller than abefore-shifting gear ratio by a predetermined value and a time pointwhen the present gear ratio reaches a second gear ratio that is set to avalue larger than the before-shifting gear ratio by a predeterminedvalue.

[0019] If the engine races during shifting, it is possible to detect thestart of shifting when the present gear ratio reaches the second gearratio, and if the engine never races during shifting, it is possible todetect the start of shifting when the present gear ratio reaches thefirst gear ratio. In either case, it is possible to early detect thestart of shifting.

[0020] According to the fifth aspect of the present invention, theactual shift time is measured by regarding as a shift starting point theearliest time point among a time point when the present gear ratio hasrepeated decreasing by a value equal to or larger than a predeterminedunit gear ratio a predetermined number of times, a time point when thepresent gear ratio reaches a first gear ratio that is set to a valuesmaller than a before-shifting gear ratio by a predetermined value, anda time point when the present gear ratio reaches a second gear ratiothat is set to a value larger than the before-shifting gear ratio by apredetermined value.

[0021] Since the start of shifting can be detected if the present gearratio has repeated decreasing by a value equal to or larger than apredetermined unit gear ratio a predetermined number of times or if thepresent gear ratio has reached the first or second gear ratio, it ispossible to detect the start of shifting at a time point in proximity tothe actual shift starting point irrespective of whether the shiftingproceeds fast or not.

[0022] It should be noted that the above-mentioned unit gear ratio ispreferably set to a value smaller than a difference between thebefore-shifting gear ratio and the first gear ratio.

BRIEF DESCRIPTION OF DRAWINGS

[0023]FIG. 1 is a diagram showing a power train control system for avehicle according to an embodiment of the present invention;

[0024]FIG. 2 is a main flow chart showing the procedure for carrying outa shift controlling operation according the embodiment;

[0025]FIG. 3 is a flow chart showing the details of the procedure formeasuring a shift time;

[0026]FIG. 4 is a diagram useful in explaining the outline of shiftstart determination;

[0027]FIG. 5 is a diagram useful in explaining an example of shift startdetermination based on a sequential decrease in gear ratio; and

[0028]FIG. 6 is a diagram useful in explaining a problem of shift startdetermination according to a prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] The best mode of carrying out the present invention will now bedescribed in detail with reference to the drawings showing an embodimentthereof.

[0030]FIG. 1 is a diagram showing a power train control system for avehicle, in which a control device according to the embodiment isincorporated. In this power train control system, an output from anengine 10 is transmitted to a wheel 30 via an automatic transmission 15.

[0031] The engine 10 has a throttle valve 11 that is connected to anaccelerator pedal, not shown, to be opened and closed. The engine 10 iscontrolled so as to acquire a predetermined output torque by an enginecontrol unit 13 that receives a throttle valve opening (hereinafterreferred to as “throttle opening”) detected by a throttle sensor 12.

[0032] The automatic transmission 15 is comprised of a torque converter16, a shift mechanism 17 including a plurality of friction engagementelements 18, and a control valve 20 that supplies and releases oilpressure to and from the friction engagement elements 18.

[0033] The control valve 20 uses an oil pump, not shown, as its oilpressure source, and is comprised of a shift actuator 21 that decideswhether to supply hydraulic oil pressure and an oil pressure regulatingactuator 22 that regulates an oil pressure value of hydraulic oilpressure. A transmission control unit 24 controls the control valve 20.

[0034] More specifically, the throttle sensor 12, an input revolutionsensor 25, that detects an input shaft revolutionary speed Vin of theshift mechanism 17, and an output shaft revolution sensor 26 thatdetects an output shaft revolutionary speed Vout of the shift mechanism17 are connected to the transmission control unit 24, which controls thecontrol valve 20 according to information received from those sensors.The transmission control unit 24 selectively engages and disengages thefriction engagement elements 18 according to the driving conditions of avehicle to thus change a plurality of gear positions.

[0035] It should be noted that a vehicle speed Vs is found bymultiplying the output shaft revolutionary speed Vout of the shiftmechanism 17 by a predetermined coefficient.

[0036] Although not illustrated, the shift actuator 21 has a pluralityof solenoids and shift valves, and changes the shift valves by turningon and off the solenoids to decide whether or not the hydraulic oilpressure will be supplied. The oil pressure regulating actuator 22 has asolenoid which is duty cycle controlled and a regulator valve, andapplies a pilot pressure, which is obtained by controlling an on-offtime ratio of an orifice by the solenoid, to the regulator valve to thusregulate an oil pressure. The arrangements of the shift actuator 21 andthe oil pressure regulating actuator 22 are identical with those ofcomponents constituting a control valve in a conventional transmission.

[0037] When changing the gear positions, the transmission control unit24 operates the shift actuator 21 and the oil pressure regulatingactuator 22 while learning-controlling a shift time T so as to ensuresmooth shifting by controlling the engagement or disengagement of thefriction engagement elements 18 in a predetermined shift time (referenceshift time) To.

[0038] According to the present embodiment, the start of shifting isdetected by determining not only whether or not the gear ratio hasreached a predetermined shift start determination gear ratio but alsoother changing conditions of the gear ratio so that thelearning-controlled shift time T can be detected at a point in proximityto an actual shift starting point.

[0039]FIG. 2 is a main flow chart showing the procedure for carrying outa shift controlling operation by the transmission control unit 24.

[0040] First, the throttle opening TV is read from the throttle sensor12 in a step 101, and the vehicle speed Vs is read in a step 102. Itshould be noted that the vehicle speed is found by reading the outputshaft revolutionary speed Vout from the output revolution sensor 26 asstated previously.

[0041] In a step 103, a target gear position is determined based on thethrottle opening TV and the vehicle speed Vs according to apredetermined shift pattern.

[0042] In a step 104, whether the shifting is necessary or not isdetermined by comparing the target gear position determined in theprevious step with the present gear position. If the target gearposition corresponds to the present gear position, it is determined thatthe present gear position will be maintained and the program proceeds tothe step 101.

[0043] If the shifting is necessary, the type of shifting i.e. fromwhich gear position to which gear position is identified in a step 105,and the previous shift time Tb in the identified type of shifting storedby learning is compared with the predetermined reference shift time Toin a step 106.

[0044] The reference shift time To has a predetermined timewidth, and ifthe previous shift time Tb lies in this time width, the program proceedsto a step 107 wherein an oil pressure correction coefficient K describedlater is maintained at K. If the previous shift time Tb is longer thanthe reference shift time To, the correction coefficient K is changed toK+ in a step 108, and if the previous shift time Tb is shorter than thereference shift time To, the correction coefficient K is changed to K−in a step 109.

[0045] After the correction coefficient is found, a basic line pressureP in shifting is calculated based on the throttle opening TV and thevehicle speed Vout in a step 110, and the correction coefficient K isadded to the basic line pressure P to find an optimum oil pressure PB ina step 111.

[0046] Then, an oil pressure command for forming the optimum oilpressure PB is outputted to the oil pressure regulating actuator 22 in astep 112, and a shifting command S is outputted to the shift actuator 21in a step 113.

[0047] In a next step 114, the present shift time T is measured, and ina step 115, the measured shift time is stored as the previous shift timeTb for the next shifting.

[0048] The program then returns to the step 101 to repeat the abovedescribed steps.

[0049]FIG. 3 is a flowchart showing the details of the procedure formeasuring the shift time in the step 114.

[0050] First, in a step 201, a sequential number count n representingthe number of sequential decreases in gear ratio as described later isset to 0.

[0051] In a step 202, the input shaft revolutionary speed Vin and theoutput shaft revolutionary speed Vout in the shift mechanism 17 are readto calculate the present gear ratio G (=Vout/Vin).

[0052] In a next step 203, it is determined whether or not the presentgear ratio G has reached a shift start determination gear ratio (thefirst gear ratio) G1, which is set to a value smaller than abefore-shifting gear ratio Gx by a predetermined value as shown in FIG.4.

[0053] If the present gear ratio G has reached the shift startdetermination gear ratio G1, the program proceeds to a step 209, and ifnot, the program proceeds to a step 204.

[0054] In the step 204, it is determined whether or not the present gearratio G has reached a racing gear ratio (the second gear ratio) G2,which is set to a value larger than the before-shifting gear ratio Gx asshown in FIG. 4 by a predetermined value. If the present gear ratio Ghas reached the racing gear ratio G2, the program proceeds to the step209, and if not, the program proceeds to a step 205.

[0055] In the step 205, it is determined whether or not a predeterminedperiod of time Tm has elapsed since the output of the shifting commandS. If the predetermined period of time Tm has elapsed, the programproceeds to the step 209, and if not, the program proceeds to a step206.

[0056] In the step 206, it is determined whether or not the present gearratio G has decreased from the previous value by a value equal to orlarger than a predetermined unit gear ratio ΔG. If the present gearratio G has decreased from the previous value, the sequential numbercount n is increased by 1 in a step 207.

[0057]FIG. 5 is an enlarged diagram showing the gear ratio between atime point when the shifting command S is outputted and a time pointwhen the shift start determination gear ratio G1 is reached. The unitgear ratio ΔG is set to a value that is single-digit smaller than adifference between the before-shifting gear ratio Gx and the shift startdetermination gear ratio G1.

[0058] In a step 208, it is determined whether or not the sequentialnumber count n has reached a predetermined value N. If the sequentialnumber count n has reached the predetermined value N, it is determinedthat the shifting has been started and the program proceeds to the step209. In the step 209, measuring of the shift time is started.

[0059] On the other hand, it is determined in the step 206 that thepresent gear ratio G has increased or that the present gear ratio G hasdecreased but is still smaller than the unit gear ratio ΔG, thesequential number count n is reset to 0 in a step 212 and the programthen returns to the step 202.

[0060] After the measuring of the shift time is started in the step 209,it is repeatedly determined in a step 210 whether or not the presentgear ratio G has reached a shift end determination gear ratio (the thirdgear ratio) G3, which is set to a value larger than an after-shifting(target gear position) gear ratio Gy by a predetermined value as shownin FIG. 4.

[0061] If the present gear ratio G has reached the shift enddetermination gear ratio G3, the measuring of the shift time isterminated in a step 211. The measured shift time T is stored as Tb inthe step 115 in the main flow chart as described above.

[0062] Among the above described steps, the steps 105 to 115 constitutea shift time learning control means of the invention, the step 202constitutes a gear ratio calculating means, and the steps 203 to step211 constitutes a shift time measuring means.

[0063] It should be noted that the order of the steps 203 to 205 and thesteps 206 to 208 may be changed alternately in the procedure.

[0064] As a result of the above described steps, if the present gearratio G has reached the shift start determination gear ratio G1 afterthe output of the shifting command S as shown in FIG. 4, this time pointts1 is regarded as the shift starting point as in the prior art. If thegear ratio G then crosses the shift end determination gear ratio G3 at atime point te1, a period of time from the time point ts1 to te1 isregarded as the shift time T.

[0065] If the present gear ratio G has increased to reach the racinggear ratio G2 although it has not yet reached the shift startdetermination gear ratio G1, this time point ts2 is regarded as theshift starting point since the present gear ratio G changes from thebefore-shifting gear ratio Gx. If the gear ratio then crosses the shiftend determination gear ratio G3 at a time point te2, a period of timefrom the time point ts2 to te2 is regarded as the shift time T.

[0066] Further, if the gear ratio has decreased sequentially apredetermined number of times by the unit gear ratio AG or more as shownin FIG. 5, this time point ts4 is regarded as the shift starting point.If the gear ratio G then crosses the shift end determination gear ratioG3 at a time point te4 (not illustrated), a period of time from the timepoint ts4 to te4 is regarded as the shift time T.

[0067] If the start of shifting is not detected in the above describedmanners, a time point ts3 at which the predetermined period of time Tmhas elapsed since the output of the shifting command S is regarded asthe shift starting point, and if the gear ratio G then crosses the shiftend determination gear ratio G3 at a time point te3, a period of timefrom the time point ts3 to te3 is regarded as the shift time T.

[0068] According to the prevent embodiment constructed in theabove-mentioned manner, the unit gear ratio ΔG is set to a value that issufficiently smaller than a difference between the shift startdetermination gear ratio G1, which is conventionally set for the purposeof detecting the start of shifting, and the before-shifting gear ratioGx, and it is determined whether or not the gear ratio G has repeateddecreasing by a value equal to or larger than a predetermined unit gearratio a predetermined number of times. Therefore, if the gear ratio Gchanges slowly, the start of shifting can be detected early at the timepoint ts4 that is much closer to the actual shift starting point ts thanthe time point ts1 when the gear ratio G decreases to the shift startdetermination gear ratio G1, as is clear from FIG. 5. of course, sinceit is also determined whether or not the gear ratio G has reached theshift start determination gear ratio G1, if the gear ratio decreasesrapidly, the start of shifting is detected when the gear ratio G reachesthe shift start determination gear ratio G1 without waiting for the gearratio G to repeat decreasing a predetermined number of times by the unitgear ratio ΔG or more.

[0069] If the engine races, it takes much time for the gear ratio G toreach the shift start determination gear ratio G1. However, since it isdetermined whether or not the gear ratio G has reached a racing gearratio G2 according to the present embodiment, the start of shifting canbe detected earlier at the time point ts2 compared with the prior art inwhich the start of shifting is detected only by determining whether ornot the gear ratio has reached the shift start determination gear ratio.

Industrial Applicability

[0070] The above described automatic transmission for vehicle accordingto the present invention is capable of finding the actual shift timewith high accuracy and using it for learning control to thus properlyengage and disengage the friction engagement elements and prevent ashift shock even in the case where the gear ratio changes at anextremely low rate at the start of shifting or in the case where theengine races.

1. A control device of an automatic transmission, which selectivelysupplies hydraulic oil pressure to a plurality of friction engagementelements to change a plurality of gear positions by engaging anddisengaging the friction engagement elements in combinations, saidcontrol device comprising: an oil pressure regulating actuator forregulating the oil fluid pressure; shift time learning control means forperforming shift time learning control by comparing a target shift timeand an actual shift time to thereby control said oil pressure regulatingactuator; and wherein said shift time learning control means comprisesgear ratio calculating means for finding a present gear ratio, and shifttime measuring means for measuring the actual shift time by determiningthat shifting is started if the present gear ratio has repeateddecreasing by a value equal to or larger than a predetermined unit gearratio a predetermined number of times.
 2. A control device of anautomatic transmission, which selectively supplies hydraulic oilpressure to a plurality of friction engagement elements to change aplurality of gear positions by engaging and disengaging the frictionengagement elements in combinations, said control device comprising: anoil pressure regulating actuator for regulating the hydraulic oilpressure; shift time learning control means for performing shift timelearning control by comparing a target shift time and an actual shifttime to thereby control said oil pressure regulating actuator; andwherein said shift time learning control means comprises gear ratiocalculating means for finding a present gear ratio, and shift timemeasuring means for measuring the actual shift time by regarding as ashift starting point an earlier time point between a time point when thepresent gear ratio has repeated decreasing by a value equal to or largerthan a predetermined unit gear ratio a predetermined number of times anda time point when the present gear ratio reaches a first gear ratio thatis set to a value smaller than a before-shifting gear ratio by apredetermined value.
 3. A control device of an automatic transmission,which selectively supplies hydraulic oil pressure to a plurality offriction engagement elements to change a plurality of gear positions byengaging and disengaging the friction engagement elements incombinations, said control device comprising; an oil pressure regulatingactuator for regulating the hydraulic oil pressure; shift time learningcontrol means for performing shift time learning control by comparing atarget shift time and an actual shift time to thereby control said oilpressure regulating actuator; and wherein said shift time learningcontrol means comprises gear ratio calculating means for finding apresent gear ratio, and shift time measuring means for measuring theactual shift time by regarding as a shift starting point a time pointwhen the present gear ratio reaches a second gear ratio that is set to avalue larger than a before-shifting gear ratio by a predetermined value.4. A control device of an automatic transmission, which selectivelysupplies hydraulic oil pressure to a plurality of friction engagementelements to change a plurality of gear positions by engaging anddisengaging the friction engagement elements in combinations, saidcontrol device comprising: an oil pressure regulating actuator forregulating the hydraulic oil pressure; shift time learning control meansfor performing shift time learning control by comparing a target shifttime and an actual shift time to thereby control said oil pressureregulating actuator; and wherein said shift time learning control meanscomprises gear ratio calculating means for finding a present gear ratio,and shift time measuring means for measuring the actual shift time byregarding as a shift starting point an earlier time point between a timepoint when the present gear ratio reaches a first gear ratio that is setto a value smaller than a before-shifting gear ratio by a predeterminedvalue and a time point when the present gear ratio reaches a second gearratio that is set to a value larger than the before-shifting gear ratioby a predetermined value.
 5. A control device of an automatictransmission, which selectively supplies hydraulic oil pressure to aplurality of friction engagement elements to change a plurality of gearpositions by engaging and disengaging the friction engagement elementsin combinations, said control device comprising: an oil pressureregulating actuator for regulating the hydraulic fluid pressure; shifttime learning control means for performing shift time learning controlcomparing a target shift time and an actual shift time to therebycontrol said oil pressure regulating actuator; and wherein said shifttime learning control means comprises gear ratio calculating means forfinding a present gear ratio, and shift time measuring means formeasuring the actual shift time by regarding as a shift starting pointthe earliest time point among a time point when the present gear ratiohas repeated decreasing by a value equal to or larger than apredetermined unit gear ratio a predetermined number of times, a timepoint when the present gear ratio reaches a first gear ratio that is setto a value smaller than a before-shifting gear ratio by a predeterminedvalue, and a time point when the present gear ratio reaches a secondgear ratio that is set to a value larger than the before-shifting gearratio by a predetermined value.
 6. A control device of an automatictransmission according to claim 2 or claim 5, wherein the unit gearratio is set to a value smaller than a difference between thebefore-shifting gear ratio and the first gear ratio.